In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, device-to-device (D2D) communications has recently been proposed as an underlay to cellular communications networks as a means to take advantage of the proximity of communicating devices and at the same time to allow the communicating devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communication shares the same spectrum as the cellular communications network, for example by reserving some of the cellular uplink resources for D2D purposes.
D2D communication is as such known in the art and a component of existing wireless technologies, including ad hoc and cellular networks. Examples of D2D communication based techniques include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These D2D based communication systems operate in unlicensed spectrum.
D2D communications is currently being defined for Release 12 (Rel-12) of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). A range of services have been identified, which can be provided by the 3GPP system based on user equipment (i.e., communicating device) being in proximity to each other.
Two ways to utilize the D2D applications communication link are direct discovery and direct communication. In both cases, the transmitting communicating device sends D2D signals that should be directly received at least by the intended receiving communicating device. Additional applications include relaying, where a communicating device relays data received from a network infrastructure or a communicating device to another communicating device, or vice-versa. Some services which may benefit from such D2D communication are commercial services and Public Safety.
Allocating dedicated spectrum for D2D purposes may be regarded as a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services could be more flexible and could provide higher spectrum efficiency. In terms of the physical layer, the Rel-12 D2D link operates in uplink spectrum (in the case of Frequency-Division Duplex, FDD) or uplink sub-frames (in the case of Time-Division Duplex, TDD). A D2D signal and wide area network signal can be multiplexed on a given carrier using Time Division Multiplexing (TDM).
D2D based communication networks should also be able to operate in multi-carrier scenarios where the cellular communications network and/or the D2D network is/are configured to operate on multiple carriers. Such carriers do not necessarily belong to a single network operator and are not necessarily coordinated and synchronized.
3GPP LTE has been investigated as a competitive radio access technology for efficient support of Machine-Type Communication (MTC). Some MTC use cases relate to devices being deployed deep inside buildings, which would require coverage enhancement in comparison to the defined coverage of the existing cellular communications network. However, it may be efficient for network operators to be able to serve MTC user equipment using already deployed radio access technology.
3GPP LTE Rel-12 has defined a user equipment power saving mode, allowing long battery lifetime and a new user equipment category allowing reduced modem complexity. In 3GPP LTE Rel-13, further development of MTC may further reduce user equipment cost and provide coverage enhancement. One element to enable cost reduction is to introduce a reduced user equipment radio frequency bandwidth of about 1.4 MHz in the downlink and uplink within any network bandwidth. Lowering the cost of MTC user equipment is a further enabler for implementation of the concept of “internet of things” (IoT). MTC user equipment used for many applications will require low operational power consumption and are expected to communicate with infrequent small burst transmissions.
As a proposal for low power, low complexity MTC communication, the MTC devices may communicate with a relay node by using LTE D2D communication. The relay node may then communicate with a radio access network node (such as an eNodeB) of the cellular communications network (such as LTE). One advantage of using such an approach is that a coverage enhancement (as required in Rel-13) can be reached, as well as some of the MTC complexity being moved to a single relay node. Hence, by using D2D communication via the relay node (that may act as a relay for several MTC devices) a LTE based capillary network structure can be built.
However, D2D links as defined in 3GPP Rel-12 are not defined to support MTC devices.
Hence, there is still a need for an improved synchronization in wireless D2D based communications networks.